7. Sloan was party to a plot that required the cooperation of a. one Starfleet Admiral and b. a high-ranking Romulan intelligence agent.
It comes from the seventh season episode, "Inter Arma Silent Leges," which features the only intelligence operation we ever see Section 31 perform. Sloan manipulates Doctor Bashir and Admiral Ross to place--spoilers, BTW--a high-ranking Federation agent into the highest levels of Romulan government.
Item 7a is easier to prove than the self-evident manipulation of Bashir. Admiral Bill Ross is a dull as all the rocks in a Zen garden. He's awful. I could convince Bill Ross that I'm the agent of a massive conspiracy which violates the principles of The Federation, and I'm from 250 years ago! The man's credulous and having him snookered requires no suspension of disbelief or further explanation.
Item 7b is a real killer. Toval is the leader of the ruthless Tal Shiar intelligence agency. He's apparently a Federation sympathizer who is feeding intelligence to Starfleet. He cooperates with Sloan to undermine a political rival. He then pretends to interrogate and shoot Sloan to cover the human's escape.
If Sloan is "just one guy" Toval would likely cooperate with him because he's an outside actor who would lend credibility to the operation to discredit his rival, Cretak. Why would Toval not have Sloan killed though? Why would he give information to The Federation? He's the opposite of Ross in that he's just way too sharp to fall for the Section 31 bit.
It's easy to believe that Toval intended to kill Sloan, wrapping up a loose end. If Sloan could have beamed away without Toval's help, I'm not sure what difference a few disruptor settings would have made; Toval doesn't need to be in on it. If the Romulan was convinced Sloan trusted him to provide an escape route, an independent escape wouldn't even be an avenue of exploration during Sloan's (very real) interrogation under a Romulan mind probe. Sloan goes along with Toval's escape plan, Toval doesn't ask about it under interrogation, and Sloan beams himself away during Toval's sudden but inevitable betrayal.
That's even assuming Sloan didn't die.
Yeah, it's none.
But Sloan isn't dead until he dies in "Extreme Measures." How could he have died months earlier?I'm glad you asked that question, because every answer to that question is more credible than a secret organization that's been totally undetected by Starfleet for over 200 years.
Clones are a thing. A clone could even behave convincingly under a mind probe with the help of false memories from a dataport, a common piece of technology that a black ops agent would be expected to have.
Transporter duplicate. Lemme tell you, after a transporter duplicate nearly started a war with Cardassia, I think they're common knowledge. What better way to feign death than to actually die and still live as a perfect copy of yourself?
You don't even need space-age technobabble to explain it. Triplets. There could be three Sloans. In fact, the "31" in "Section 31" could stand for three-in-one, a trinity of brothers (I've been reading Campbell; give me a break). Twins would be slightly more likely, but why not dream big? This does technically undermine the "just one guy" theory, but I don't think it undermines the spirit. It also explains how Sloan could get from Romulus to Deep Space Nine without a starship; because he never left.
Oh, right, but Toval was also feeding intelligence to Starfleet. Ignoring the fact that he'd be offering Starfleet intelligence that was either already being shared (Dominion) or useless (Klingon), consider:
a) We learn of Toval's intelligence from Admiral Ross.
b) Admiral Ross is not in Starfleet intelligence.
c) Random admirals are not informed of high-level intelligence sources.
d) As always, Ross is a fucking idiot.
I posit that Admiral Ross learned of Toval's "intelligence" from Sloan. In actuality Toval has given The Federation nothing. He gets added to the Continuing Committee, kills Sloan, and continues to argue for the abrogation of the alliance. Neat. Simple. Likely.
Finally, think of Section 31's side. When either Cretak or Toval would continue Romulan support of the war against The Dominion--an existential threat to The Federation--the benefit of having Toval in place post-war to deal with the persistent nuisance of The Romulan Star Empire is in no way worth the consequences of botching an intelligence job on Romulan soil. Y'know, a thing that would likely cause the Romulans to drop out of the war.
It's a dumb, rookie move, a sign of a pretender with delusions of grandeur trying to play in the big leagues. Sloan can manipulate Federation doctors in a closed environment, program holodecks, hack computers, and break into places, but a few minor successes don't make a rank amateur into someone who can go toe-to-toe with the Tal Shiar.
Is it not more likely that Sloan mistook having a tiger by tail with grabbing a viper's ass? That he was barely smart enough to have an out when Toval decided to wrap up his last loose end?
What's more likely, that after 200 years of complete secrecy, Section 31 revealed their presence to the Romulans for a single operation, or that Sloan is an ambitious, rogue operator who was easily captured by a professional intelligence agency?
For fucks sake, Section 31 is just one guy.